The present invention relates to a process for the catalytic production of methanol and to a unit for running the process. Similar processes for the catalytic production of methanol were presented over the past years, patent numbers DE 21 17 060, DE 25 29 591, DE 32 20 995, DE 35 18 362, U.S. Pat. No. 2,904,575 and DE 41 00 632 being referred to as examples for the great variety of technical solutions.
Technologies of this kind generally use a suitable synthesis gas which predominantly contains constituents such as carbon monoxide, carbon dioxide and hydrogen, but also minor amounts of vapour, nitrogen, methane, ammonia, ethene, ethyne, hydrogen cyanide, oxygen, sulphur compounds, chlorine compounds, iron compounds, especially iron carbonyls, elementary carbon, especially carbon black particles, metal compounds of vanadium, potassium, sodium and nickel as well as solid particles in the form of dust. Such a synthesis gas is usually fed at a pressure of approx. 40 to 100 bar to a reaction system of several reactors which are arranged either in a cascade and/or in a loop system, as a rule being provided with a catalyst inventory and a device for heat dissipation. In each of the reactors, a partial conversion to methanol is achieved at a temperature of more than 200° C., said methanol being condensed and purified downstream of the respective reactor. The reaction heat released from the catalyst bed of the reactors is utilised for the production of steam which can be used for other applications, tube bundles being provided for this purpose in the catalyst bed and being fed with boiler feed water that evaporates in these pipelines.
The catalysts selected are special copper-based types but they involve the disadvantage to be very expensive, extremely sensitive to catalytic poison and to trigger an increasing number of secondary reactions at high temperatures which produce ethanol, butyl alcohol and dimethyl ether, and under relatively high working temperatures, the copper-based crystallites will act catalytically and cause a rearrangement to form larger crystals of a smaller specific surface and thus reduce the catalyst activity specific to space.
The engineer commissioned to construct a plant for methanol production is therefore confronted with the problem to ensure long service life for the reactors to meet the rated methanol production capacity by adequately dimensioning the catalyst volume and by providing sophisticated equipment for the removal of catalyst poison from the synthesis gas. This is usually achieved by calculating the required catalyst volume with a wide safety margin which accordingly increases the specific investment cost.
The operator of a plant for the methanol production is consequently confronted with the problem that the activity of the catalyst is subject to considerable variations throughout the service life of the catalyst. At the beginning of a production cycle, the fresh catalyst material packed is very active and requires to permanently counteract the danger of a reaction beyond control, i.e. by operating the steam generation, which moderates the reaction, at a lower temperature and a lower pressure which will result in a deterioration of the steam quality. An other option is to perform an adequate synthesis gas rarefaction using inert components, but this involves the disadvantage of higher equipment costs for the compression. If the methanol synthesis took place without being adequately moderated, the temperatures in the reactors would rise to such an extent that it might stimulate the formation of secondary products and cause faster catalyst ageing due to crystallite rearrangement.
As the production cycle proceeds, the catalyst layers next to the synthesis gas are gradually poisoned due to the existing traces of catalyst poison. Such poisoning takes place in different ways: On the one hand, sulphur and chlorine compounds as well as ammonium will cause a chemical deactivation of the catalytically active catalyst components. On the other hand, solid particles deposit on the surface of the catalyst and thus form a layer which inhibits diffusion. In addition, iron or nickel carbonyl compounds as well as other metal compounds liberated for example by corrosion (rust) or abrasion from the pipelines or other plant construction materials may change the catalytic system itself and thus contribute to the catalysis of other undesired end products. Since all of these poisonous effects are irreversible, the whole catalyst packing cannot be regenerated and must be disposed of after use.
In a first step, the plant operator may counteract the gradual poisoning by raising the partial pressure of the components involved in the reaction and in a further step he may increase the overall pressure, which in turn will result in the need for additional compression capacity; and in a final step he may raise the reaction temperature by augmenting the pressure and temperature of the moderating steam. The final step, however, accelerates the ageing process of said part of the catalyst bed that was not poisoned before and thus produces the undesired secondary products mentioned above. Hence, it has been of primary interest for years that a solution to this problem be provided by plant construction companies to the benefit of plant operators.
The aim of the present invention is to overcome the described problems by a cost-efficient technological solution.